These are remedies which produce vomiting.

Action. - The act of vomiting consists in compression of the stomach by the simultaneous spasmodic contraction of the diaphragm and abdominal muscles, while at the same time relaxa-tion of its cardiac orifice is produced by contraction of the fibres which radiate out from the lower end of the oesophagus along the gastric walls. By their contraction these fibres draw the stomach up towards the diaphragm and pull the walls of the oesophagus apart at its lower end so as to open the cardia. When the cardiac orifice dilates at the same moment that the stomach is compressed between the diaphragm and the abdominal muscles, its contents are expelled and vomiting occurs; but when the compression of the stomach and dilatation of the cardiac orifice do not take place simultaneously, the contents of the stomach are retained and the efforts are then termed retching.

The nerve-centre which regulates the movements of vomiting is situated in the medulla oblongata. The movements of vomiting are modified respiratory actions, and the respiratory centre appears to be closely connected with the vomiting centre. Indeed some groups of ganglion cells probably take part both in respiration and vomiting, or in other words form part of both the respiratory and vomiting centres (Fig. 80, p. 235).

The reason for this supposition is not merely that the movements of vomiting consist of modified respiratory movements, but that drugs which cause vomiting also increase the respiratory activity. Emetics usually quicken the respiration considerably before they produce vomiting, and if injected into the veins they not only quicken the respiration, but prevent the condition of apnoea being produced by vigorous artificial respiration.

On the other hand, the desire to vomit may be lessened tosome extent by taking frequent and deep inspirations, and narcotics which diminish the excitability of the respiratory centre also lessen the tendency to vomit.

The motor impulses from the vomiting centre are sent to the abdominal muscles, diaphragm, stomach and oesophagus by the intercostal, phrenic, and vagus nerves respectively. Section of the vagi generally, though not always, destroys the power to vomit, because it disturbs the co-ordination of the cardia and the abdominal muscles and diaphragm, so that they no longer act simultaneously, and vomiting does not occur, although retching may continue.

The vomiting centre is usually excited by stimulation of afferent nerves passing upwards to it from the body, or by impulses sent down to it from the brain.

The brain may be stimulated so as to act on the vomiting centre in the medulla through impressions on the nerves of special sense, such as a disgusting sight, stench, or taste, or by the recollection of such subjects. Irritation of the brain itself or of its membranes by inflammation, tubercle, haemorrhage, softening, or cancer may also excite vomiting. The afferent nerves are shown in the accompanying diagram (Fig. 130). Those chiefly concerned with the action of emetics are :1. Branches of the glosso-pharyngeal nerve to the soft palate, the root of the tongue, and the pharynx. Tickling these parts with the finger or with a feather is one of the readiest means of inducing vomiting. Vomiting also occurs when the soft palate, tonsils, or pharynx are inflamed, especially in children.

2. The nerves of the stomach. These are chiefly branches of the pneumogastric, but they are contained also in the sympathetic system.

3. Mesenteric nerves causing vomiting in hernia.

4. Nerves of the liver and gall-duct.

5. Nerves of the kidney and ureter.

6. Vesical nerves.

7. Uterine nerves.

8. Pulmonary branches of the vagus causing vomiting in phthisis.

There are also a number of other nerves which produce vomiting, but are more important in connection with pathological vomiting than with the action of emetics.

When less was known regarding the action of the nervous system in vomiting, Emetics were divided, according to their relation to the stomach, into direct and indirect. Direct emetics were those which acted only when introduced into the stomach. Indirect were those which acted when injected into the blood.

Their relation to the vomiting centre is of course the reverse. Drugs which are applied directly to the stomach act reflexly or indirectly on the vomiting centre, while those injected into the blood may be carried by the circulation to the medulla and act directly upon it.

It is to be noted, however, that drugs injected into the circulation are carried not only to the nerve-centres but to the stomach, action.

Fig. 130.   Diagram showing the afferent nerves by which the vomiting centre may be excited to and may be excreted by the gastric mucous membrane.

Fig. 130. - Diagram showing the afferent nerves by which the vomiting centre may be excited to and may be excreted by the gastric mucous membrane. They may thus irritate the afferent nerves of the stomach and stimulate the vomiting centre reflexly just as they do when given by the mouth. Thus it has been shown by Brinton that tartar emetic injected into the veins of a dog is excreted in a few minutes into the stomach, and may be found on testing its contents.

It is therefore evident that the action of drugs in causing vomiting may be complex, and that drugs injected into the blood or under the skin may cause vomiting, both by (1) irritating the vomiting centre in the medulla directly when conveyed to it by the circulation; and (2) by irritating it reflexly from the stomach, whither they have also been conveyed by the blood.

It is frequently very difficult to determine in which of these two ways a drug has acted, and sometimes almost impossible to decide with certainty.

The reasons for believing that any drug injected into the circulation has caused vomiting by irritating the medulla reflexly through the stomach, and not by acting directly upon it, are:

(a) When the vomiting does not take place immediately on injection, but only after sufficient time has elapsed to allow of excretion of the drug into the stomach.

(b) When the quantity of a drug required to produce vomiting by injection into the veins is greater than that which is sufficient to produce a similar effect if introduced into the stomach. It is probable that some drugs, as tartar emetic, act in both ways, because, as has already been mentioned, it is excreted into the stomach and will there act as an irritant.

But it will also produce vomiting when the stomach has been excised and replaced by a bladder, as in Magendie's celebrated experiment. Even this experiment, however, does not prove that tartar emetic acts directly on the vomiting centre, inasmuch as it is possible that it may be excreted by the oesophagus or intestines and irritate the vomiting centre reflexly through them. As tartar emetic, however, appears to act as an irritant chiefly in those parts of the body where there is an acid secretion, it seems doubtful whether it would produce irritation in the oesophagus and intestines such as it does in the stomach. It therefore seems not improbable that the vomiting which it occasions after excision of the stomach is due to its direct action on the medulla oblongata, but this cannot be regarded as quite proved. In order to avoid the confusion which the terms direct and indirect emetics are likely to produce with regard to their relations to the stomach and vomiting, it is better to describe them as, and to employ the terms, topical or local and general emetics.

Topical, or local, are such as produce vomiting by acting locally on the pharynx, oesophagus, or stomach; and general, such as act through the medium of the circulation. The line between the two is not distinct, inasmuch as tartar emetic will produce vomiting in either way, and so will sulphate of zinc, or sulphate of copper. The local action of sulphate of zinc and sulphate of copper, however, on the stomach is so much greater than their general action that they may be classed among the local emetics.